BiologyCBSE Class 12 · Microbes in Human Welfare

Microbes as Biofertilisers

Updated 15 October 2026 · Fact-checked

Biofertilisers are organisms that enrich the nutrient quality of soil. Rhizobium in legume root nodules and free-living Azotobacter and Azospirillum fix atmospheric nitrogen; mycorrhizal fungi such as Glomus absorb phosphorus for plants; and cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix nitrogen and add organic matter, especially in paddy fields.

Understand Microbes as Biofertilisers

Why biofertilisers. Heavy use of chemical fertilisers has caused considerable environmental pollution, so farmers are moving towards organic farming and biofertilisers. Biofertilisers are living organisms that raise the nutrient quality of soil; they come mainly from three groups: bacteria, fungi and cyanobacteria.

1. Bacteria.

*Rhizobium* forms a symbiotic association with the roots of leguminous plants, producing root nodules. The bacteria fix atmospheric nitrogen into organic forms that the plant uses as a nutrient.
Free-living soil bacteria such as *Azospirillum* and *Azotobacter* also fix atmospheric nitrogen, enriching the soil's nitrogen content (no nodules needed).

2. Fungi: mycorrhiza. Fungi form symbiotic associations with plant roots called mycorrhiza; many members of the genus *Glomus* do this. The fungal partner absorbs phosphorus from the soil and passes it to the plant. Plants with mycorrhiza also show:

resistance to root-borne pathogens;
tolerance to salinity and drought;
an overall increase in plant growth and development.

The plant, in return, supplies the fungus with food (carbohydrates), so both benefit.

3. Cyanobacteria (blue-green algae). These autotrophic microbes are widespread in water and on land, and many fix atmospheric nitrogen: *Anabaena, Nostoc, Oscillatoria*. In paddy fields, they are an important biofertiliser. They also add organic matter to the soil and increase its fertility.

In practice. A number of biofertilisers are available commercially in India, and farmers use them regularly to replenish soil nutrients and reduce dependence on chemical fertilisers.

Summary table.

Biofertiliser	Group	Lifestyle	Benefit
Rhizobium	bacterium	symbiotic, root nodules of legumes	N₂ fixation
Azotobacter, Azospirillum	bacteria	free-living in soil	N₂ fixation
Glomus (mycorrhiza)	fungus	symbiotic with roots	P absorption; pathogen resistance; salt and drought tolerance; growth
Anabaena, Nostoc, Oscillatoria	cyanobacteria	free-living, autotrophic (paddy fields)	N₂ fixation; organic matter

Biofertilisers vs chemical fertilisers.

Renewable, living, multiply in soil vs manufactured, must be bought and applied repeatedly.
Improve soil structure and organic matter (cyanobacteria) vs can harm soil quality with overuse.
Little pollution vs run-off pollutes water bodies.
Slower, steady supply vs quick supply.

Flow chart (nitrogen): atmospheric N₂ → fixed by Rhizobium (nodules), Azotobacter/Azospirillum (soil) or cyanobacteria (paddy water) → organic/usable nitrogen → plant growth.
Flow chart (phosphorus): soil phosphorus → absorbed by Glomus hyphae → passed to root → plant growth (plant returns sugars to fungus).

Draw and label: legume root with nodules and mycorrhiza (simple sketch). Checklist: a root system with small round swellings on lateral roots; fine fungal threads around and into another root. Labels: root nodules (Rhizobium), lateral root, fungal hyphae (mycorrhiza, Glomus), soil.

Microbes as Biofertilisers: key facts

Nitrogen fixers: $R hi z o bi u m (symbiotic), A z o t o ba c t er, A z os p i r i l l u m (free-living), cyanobacteria$; All fix atmospheric N₂.
Mycorrhiza: $Gl o m u s + root \to P uptake, pathogen resistance, salt and drought tolerance$; Mutualism: fungus gets food from the plant.
Cyanobacteria: $A naba e na, N os t oc, O sc i l l a t or ia \to N_{2} fixation + organic matter$; Important in paddy fields.
Definition: $biofertiliser = organism that enriches soil nutrient quality$; Sources: bacteria, fungi, cyanobacteria.
Goal: $replenish soil nutrients + reduce chemical fertiliser use$; Part of organic farming.

How to answer Microbes as Biofertilisers questions

Questions ask for examples, mechanisms or a recommendation for a crop or soil. Match the nutrient problem to the right organism.

1Identify the nutrient need: nitrogen or phosphorus (or general fertility/organic matter).
2For nitrogen in legumes, choose Rhizobium (root nodules); for non-legume crops, free-living Azotobacter or Azospirillum; for flooded paddy, cyanobacteria.
3For phosphorus or drought/salinity problems, choose mycorrhiza (Glomus).
4State the relationship: symbiotic (Rhizobium, mycorrhiza) or free-living (Azotobacter, Azospirillum, cyanobacteria).
5Give the extra benefits where relevant: mycorrhiza (pathogen resistance, salt and drought tolerance); cyanobacteria (organic matter).
6End with why biofertilisers are preferred: less pollution, renewable, reduce chemical dependence.

Microbes as Biofertilisers in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Matching biofertilisers to crops

Suppose an agriculture officer visits three farms: Farm 1 grows rice in flooded fields; Farm 2 grows groundnut (a legume); Farm 3 grows wheat in slightly saline, phosphorus-poor soil that dries quickly.

Recommend a biofertiliser for each farm and explain how it helps.

Try it first, then show the solutionSolution

Farm 1 (paddy): cyanobacteria such as Anabaena or Nostoc; they grow in the water, fix atmospheric nitrogen and add organic matter.
Farm 2 (groundnut): Rhizobium; it forms root nodules on the legume and fixes nitrogen into organic forms used by the plant.
Farm 3 (wheat): mycorrhizal fungi (Glomus) to absorb phosphorus and improve tolerance to salinity and drought; free-living Azotobacter or Azospirillum can add nitrogen.

Answer: Paddy: cyanobacteria; groundnut: Rhizobium; wheat in saline, P-poor, dry soil: Glomus mycorrhiza (plus Azotobacter for N).

Memory hook: Water rice: blue-greens; legumes: nodules; dry poor soil: fungal partners.

Tricks to remember Microbes as Biofertilisers

Short memory aids. Say each one aloud once, then use it on a question the same day.

RAA fix N
Rhizobium, Azotobacter, Azospirillum fix nitrogen.
Use it when: Bacterial nitrogen fixers.
Glomus grabs phosphorus
G-P: Glomus gets Phosphorus.
Use it when: Mycorrhiza function.
A-N-O in the paddy
Anabaena, Nostoc, Oscillatoria: blue-greens in rice fields.
Use it when: Cyanobacterial biofertilisers.

Microbes as Biofertilisers: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define and classify · 2 marks · Very short answerBoard level

What are biofertilisers? Name the three main groups of organisms used, with one example each.

Try it first, then show the solutionSolution

Biofertilisers are organisms that enrich the nutrient quality of the soil (1 mark).
Bacteria (Rhizobium), fungi (Glomus, mycorrhiza), cyanobacteria (Anabaena) (1 mark).

Answer: Soil-enriching organisms: bacteria (Rhizobium), fungi (Glomus), cyanobacteria (Anabaena).

Example 2: Mycorrhiza · 3 marks · Short answerBoard level

What is mycorrhiza? Name a genus that forms it and give three benefits to the plant.

Try it first, then show the solutionSolution

Mycorrhiza is a symbiotic association between a fungus and plant roots (1 mark).
Many members of the genus Glomus form mycorrhiza (1 mark).
Benefits: the fungus absorbs phosphorus from soil and passes it to the plant; resistance to root-borne pathogens; tolerance to salinity and drought; overall better growth (any three) (1 mark).

Answer: Fungus-root symbiosis; Glomus; phosphorus uptake, pathogen resistance, salt/drought tolerance.

Example 3: Symbiotic vs free-living · 2 marks · Very short answerHarder board level

Differentiate between Rhizobium and Azotobacter as biofertilisers.

Try it first, then show the solutionSolution

Rhizobium is symbiotic: it lives in root nodules of leguminous plants and fixes nitrogen for the host (1 mark).
Azotobacter is free-living in the soil and fixes nitrogen, enriching soil nitrogen for any crop (1 mark).

Answer: Rhizobium: symbiotic, legume nodules. Azotobacter: free-living in soil.

Example 4: Cyanobacteria in paddy · 2 marks · Very short answerHarder board level

Why are cyanobacteria especially useful as biofertilisers in paddy fields? Name two such cyanobacteria.

Try it first, then show the solutionSolution

Paddy fields are flooded, and cyanobacteria are autotrophic aquatic microbes that grow well there, fix atmospheric nitrogen and add organic matter, increasing fertility (1 mark).
Examples: Anabaena, Nostoc, Oscillatoria (any two) (1 mark).

Answer: They thrive in flooded fields, fix N₂ and add organic matter; Anabaena, Nostoc.

Example 5: Why move away from chemical fertilisers · 3 marks · Short answerHarder board level

Give three reasons why farmers are being encouraged to use biofertilisers rather than only chemical fertilisers.

Try it first, then show the solutionSolution

Chemical fertilisers have caused considerable environmental pollution; biofertilisers cause little pollution (1 mark).
Biofertilisers are living and multiply in soil, replenishing nutrients naturally and reducing dependence on purchased chemicals (1 mark).
Some, like cyanobacteria, also add organic matter and improve soil fertility; mycorrhiza adds drought, salt and pathogen tolerance (1 mark).

Answer: Less pollution, self-renewing nutrient supply, better soil health and stress tolerance.

Example 6: Designing a field trial · 3 marks · Short answerHOTS / competency

Suppose a student wants to test whether Glomus improves growth of maize in phosphorus-poor soil. Design a simple experiment, state the expected result and give one other effect she might observe if the soil is dry.

Try it first, then show the solutionSolution

Grow maize in two identical sets of pots with the same P-poor soil, water and light; inoculate one set with Glomus spores (test) and leave the other uninoculated (control) (1 mark).
Expected: inoculated plants grow taller and heavier because the fungal partner absorbs phosphorus and passes it to the roots (1 mark).
In dry soil, inoculated plants should also wilt less, since mycorrhizal plants show greater tolerance to drought (1 mark).

Answer: Inoculated vs uninoculated pots; inoculated plants grow better (more P) and tolerate drought better.

Where students lose marks in Microbes as Biofertilisers

The slips examiners see most often, and what to write instead.

Mistake: Calling Glomus a nitrogen fixer
All biofertilisers are assumed to fix nitrogen.
Wrong working
Glomus fixes atmospheric nitrogen.
Right working
Glomus (mycorrhiza) absorbs phosphorus for the plant.
Do this: Glomus grabs phosphorus.
Usually costs: 1 mark.
Mistake: Saying Azotobacter forms nodules
Symbiotic and free-living are mixed up.
Wrong working
Azotobacter forms nodules in pea roots.
Right working
Rhizobium forms nodules; Azotobacter is free-living.
Do this: Only Rhizobium in nodules (in this chapter).
Usually costs: 1 mark.
Mistake: Listing Trichoderma as a biofertiliser
Biocontrol and biofertiliser are confused.
Wrong working
Trichoderma is a biofertiliser.
Right working
Trichoderma is a biocontrol agent (against pathogens).
Do this: Biofertilisers add nutrients; biocontrol agents fight pests and pathogens.
Usually costs: 1 mark.
Mistake: Forgetting extra benefits of mycorrhiza
Only phosphorus is mentioned.
Wrong working
Mycorrhiza helps in phosphorus absorption only.
Right working
Also pathogen resistance, salinity and drought tolerance, better growth.
Do this: List all four in 3-mark answers.
Usually costs: 1 mark.

Exam tips

Classify each biofertiliser as bacterium, fungus or cyanobacterium, and symbiotic or free-living.
Always name examples with the right group: Rhizobium, Azotobacter, Azospirillum, Glomus, Anabaena, Nostoc, Oscillatoria.
Link cyanobacteria with paddy fields in answers.
Contrast biofertilisers (nutrition) with biocontrol agents (protection) when a question mixes both.
End with the reason for using them: reduce chemical fertiliser use and pollution.

Practice questions on Microbes as Biofertilisers

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is a free-living nitrogen-fixing bacterium?
(a) Rhizobium (b) Azotobacter (c) Glomus (d) Trichoderma
Hint
No nodules.
Answer
(b) Azotobacter.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Plants with mycorrhiza tolerate drought and salinity better.
Reason (R): The fungal partner absorbs phosphorus from the soil and passes it to the plant.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Are these two separate benefits?
Answer
(b). Both are true benefits of mycorrhiza, but phosphorus uptake does not by itself explain drought and salt tolerance.
Q3 · 1 mark · Very short answerBoard level
Name the organism that forms root nodules in legumes and state what it does.
Hint
Symbiotic bacterium.
Answer
Rhizobium; it fixes atmospheric nitrogen into organic forms used by the plant.
Q4 · 3 marks · Short answerHOTS / competency
Suppose a farmer grows wheat on the same field every year and applies only urea. Another farmer grows wheat one year and a legume (gram) the next, without much urea, and her soil stays fertile. Explain the second farmer's success and suggest a biofertiliser for the first.
Hint
Legume nodules.
Answer
Gram is a legume whose root nodules contain Rhizobium, which fixes atmospheric nitrogen; after the crop, the nitrogen-rich roots and residues enrich the soil for the next wheat crop, so less urea is needed. The first farmer could apply free-living nitrogen fixers such as Azotobacter or Azospirillum (and mycorrhizal fungi for phosphorus) to reduce dependence on urea.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a farmers' cooperative sells three packets: Packet A contains a blue-green microbe for rice fields; Packet B contains a fungus that forms associations with roots; Packet C contains bacteria that live in the soil without forming nodules.
(a) Give one example each for A, B and C. (1)
(b) What nutrient does B mainly supply, and what extra benefits does it give? (1)
(c) Explain two ways in which A improves paddy soil. (2)
Hint
Cyanobacteria, Glomus, Azotobacter.
Answer
(a) A: Anabaena (or Nostoc, Oscillatoria); B: Glomus (mycorrhiza); C: Azotobacter or Azospirillum. (b) Phosphorus; also resistance to root-borne pathogens, tolerance to salinity and drought, and better growth. (c) It fixes atmospheric nitrogen, adding nitrogen to the soil, and it adds organic matter, increasing soil fertility.
Q6 · 2 marks · Very short answerHarder board level
Why are biofertilisers called 'living fertilisers'? Give one limitation compared with chemical fertilisers.
Hint
They are organisms.
Answer
They are living organisms (bacteria, fungi, cyanobacteria) that multiply in soil and keep adding nutrients. Limitation: they act more slowly and depend on conditions such as moisture and temperature, so they may not meet a sudden heavy nutrient demand as quickly as chemicals.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which genus forms mycorrhiza?Show answer
Glomus (many members).
2.Name two free-living nitrogen-fixing bacteria.Show answer
Azotobacter and Azospirillum.
3.In which fields are cyanobacteria important biofertilisers?Show answer
Paddy (rice) fields.
4.Which nutrient does mycorrhiza supply?Show answer
Phosphorus.
5.Name three nitrogen-fixing cyanobacteria.Show answer
Anabaena, Nostoc, Oscillatoria.
6.What forms root nodules in legumes?Show answer
Rhizobium.

Microbes as Biofertilisers: frequently asked questions

What are biofertilisers?

Organisms that enrich the nutrient quality of the soil, mainly bacteria, fungi and cyanobacteria.

How does Rhizobium act as a biofertiliser?

It forms root nodules on leguminous plants and fixes atmospheric nitrogen into organic forms the plant can use.

What is mycorrhiza and how does it help plants?

A symbiotic association of a fungus (e.g. Glomus) with roots; the fungus absorbs phosphorus for the plant and also gives resistance to root pathogens and tolerance to salinity and drought.

Why are cyanobacteria used in paddy fields?

They grow in the flooded fields, fix atmospheric nitrogen and add organic matter, increasing soil fertility.

Why are biofertilisers better for the environment?

They replenish soil nutrients naturally and reduce the use of chemical fertilisers, which cause pollution.